Later this morning (Monday 4 July) I will be on BBC radio talking about NASA’s Juno mission to the planet Jupiter. This is the latest space probe to be sent to study the largest planet in the Solar System, and follows on the highly successful Galileo spacecraft which studied Jupiter in the 1990s.
Juno left Earth in August 2011 and is due to arrive at Jupiter today. But, in order to go into orbit about the planet a rocket needs to be fired to slow the spacecraft down and put it into orbit. This is due to happen tomorrow (Tuesday 5 July). The rocket engine which will do this was built in England. If the ‘burn’ fails, the mission will fail, as the space probe will just hurtle past Jupiter and continue on into the outer Solar System.
What are Juno’s scientific objectives?
In addition to studying Jupiter, the Galileo spacecraft spent a great deal of time studying her four large moons; Io, Europa, Ganymede and Callisto. Galileo was in an equatorial orbit. Juno, on the other hand, will be put into a polar orbit. Its main objective is to study Jupiter, rather than its moons.
Jupiter is what is known as a gas giant. It is mainly hydrogen, and contains more mass than all the other planets in the Solar System put together. In fact, it is a failed star; if it were some 10 times more massive it would have had enough mass to ignite hydrogen fusion in its core. Even though it is not burning hydrogen, it is still leaking heat left over form its collapse into a planet 4.5 billion years ago.
In the last 20 years we have discovered many Jupiter-like planets orbiting other stars. Most of these are much closer to their parent star than Jupiter is to the Sun, and this has raised questions about how gas giants can be so close to their parent star, and how is Jupiter where it is in our Solar System? Jupiter is about five times further away from the Sun than the Earth is, and much further away than the Jupiter-like planets we have found around other stars. Did Jupiter start off closer to the Sun and get kicked further out, or did it migrate inwards from further away? We don’t know.
Some of the things Jupiter hopes to determine are
- the ratio of oxygen to hydrogen in Jupiter’s atmosphere. By determining this ratio it will effectively be measuring the amount of water, which will help distinguish between competing theories of how Jupiter formed.
- the mass of the solid core believed to lie at the planet’s centre, deep below the very thick and extensive atmosphere. This also has implications for its origin.
- the internal structure of Jupiter – it will do this by precisely mapping the distribution of Jupiter’s gravitational field.
- its magnetic field to better understand its origin and how deep inside Jupiter the magnetic field is created.
- the variation of atmospheric composition and temperature at all latitudes to pressures greater than 100 bars (100 times the atmospheric pressure at sea level on the Earth).
Juno has a funded operational lifetime of about 18 months. In order to better study the interior of Jupiter, the spacecraft will plunge into the planet’s atmosphere in February of 2018, making measurements as it does so.
Juno’ rocket successfully fired at about 3:20 UT today (Tuesday 5 May) and is now in orbit about Jupiter. It will complete two large 53-day orbits before being inserted into its 14-day orbit for science operations. This 14-day orbit is highly elliptical, and at its closest the probe will come to within 4,300 km of the cloud tops.